초록
Transparent Thin Film Transistor(TTFT) is a potential factor for the advanced generation display, optoelectronic applications, and so on. Especially, ZnO has been studied for optoelectronic devices, because it has a wide band-gap (3.37 eV) and a large exciton binding energy of 60 meV over GaN (26 meV).
In this thesis, undoped ZnO films for the channel layer of TFTs and HfO2 films for the gate dielectric layer deposited by pulsed laser deposition at room temperature, respectively.
We studied each of ZnO and HfO2 films to optimize for a channel and gate dielectric layer of thin film transistor. ZnO film grown at 70 mTorr show remarkably low resistivity. Accordingly, The best condition for channel layer was that sample deposited at oxygen pressure of 70 mTorr. Because thin film transistor needed high resistivity films to achieve low off-current and high on-off ratio.
The gate insulator film was deposited at 50 mTorr, because this condition was suitable for gate dielectric characteristics. Because, HfO2 film deposited at 50 mTorr show low leakage current (8.50×10-6 A/cm2) and high relative dielectric constant value (about 26).
Staggered bottom-gate ZnO thin film transistors of micrometer scales were implemented using fully lithographic without etching process. ZnO thin film transistor with high-κ dielectric layer showed saturation mobility of 7.42 cm2/V-s, on/off ratio of 1.25×105 and subthreshold swing of 2.13 V/decade with threshold voltage of 4.14 V. The optical transmittance of the thin film transistor was above 80% at visible wavelength region.
Comparisons with low-κ thin film transistor, high-κ thin film transistor show better transistor performance owing to gate oxide layer with high relative dielectric constant. However high-κ thin film transistor indicated highly leakage current between gate dielectric and channel layer. There is a necessity to which the leakage current will be improved.
These results should increase the prospects by using stable thin film transistors for active matrix liquid crystal display (AM-LCD) and active matrix organic light emitting diode (AM-OLED).
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